Associate Laboratory of  Energy, Transports and Aeronautics

Funder

Organizational Unit

Publications

Turbulence Quantification in Supercritical Nitrogen Injection: An Analysis of Turbulence Models

Publication . Magalhães, Leandro; Carvalho, Francisco; Silva, André; Barata, Jorge M M

In Liquid Rocket Engines, higher combustion efficiencies come at the cost of the propellants exceeding their critical point conditions and entering the supercritical domain. The term fluid is used because, under these conditions, there is no longer a clear distinction between a liquid and a gas phase. The non-conventional behavior of thermophysical properties makes the modeling of supercritical fluid flows a most challenging task. In the present work, a RANS computational method following an incompressible but variable density approach is devised on which the performance of several turbulence models is compared in conjunction with a high accuracy multi-parameter equation of state. Also, a suitable methodology to describe transport properties accounting for dense fluid corrections is applied. The results are validated against experimental data, becoming clear that there is no trend between turbulence model complexity and the quality of the produced results. For several instances, one- and two- equation turbulence models produce similar and better results than those of Large Eddy Simulation (LES). Finally, considerations about the applicability of the tested turbulence models in supercritical simulations are given based on the results and the structural nature of each model.

2020-06-02Journal article

Open access

Plunging Airfoil Motion: Effects of Unequal Ascending and Descending Velocities

Publication . Rodrigues, Diana Carvalho; Camacho, Emanuel; Neves, Fernando M. S. P.; Barata, Jorge M M; Silva, A. R. R.

Biomimetics is a multidisciplinary area of science studying the development of new technologies, whose source of inspiration is Nature and has given rise to new technologies inspired by biological solutions at macro and nanoscales. Successive work carried out by researchers in this field revealed that flapping wings offers not only benefits but unique aerodynamic advantages when compared to the traditional fixed-wings, especially when approaching small scales. This work presents an experimental study concerning the created vortical structures of a plunging NACA0012 airfoil subjected to an asymmetrical motion at a Reynolds number of 1500 with different reduced frequencies and nondimensional amplitudes. The asymmetric motion studied was based on a velocity triangular wave with special focus in a plunging cycle asymmetry of 75%. Over its plunging motion, the unequal ascending and descending velocities revealed that the airfoil can produce both thrust and lift simultaneously. Leading-Edge Vortex (LEV) formation and its convection over the upper surface of the airfoil was seen as a possible power reduction mechanism which could be a way to improve propulsive and energy extraction efficiencies.

2020-06-08Conference object

Restricted access

Locally variable turbulent Prandtl number considerations on the modeling of Liquid Rocket Engines operating above the critical point

Publication . Magalhães, Leandro; Silva, A. R. R.; Barata, Jorge M M

The general idea behind the present work is to study the injection of a cryogenic liquid numerically into rocket engines, where propellant conditions are above the thermodynamic critical point, for a non-reactive case. The singular behavior of thermodynamic and transport properties at and around the critical point makes this a most challenging task. While mass diffusivity, surface tension, and latent heat are zero at the critical point, isentropic compressibility, specific heat, and thermal conductivity tend to infinity. As a result, the distinction between liquid and solid phases disappears. Ultimately, the fluid has liquid-like density and gas-like properties, mass diffusion replaces vaporization as a governing parameter, and it dominates over jet atomization. Henceforth, any model used incorporates as close as possible to reality, the variation of thermodynamic and transport properties. An incompressible variable-density flow is simulated using Favre averages (FANS) with a locally variable turbulent Prandtl number, taking into account the potential core, transition, and the self-similar region of the jet. The use of a turbulence model with a variable turbulent Prandtl number arises from the ineffectiveness in predicting observed anisotropies in the thermal eddy diffusivity fields when this value is taken as a constant. Favre averaged conservation equations for mass, momentum, and energy are coupled with the k- two-equation turbulence model and discretized following the third order upwind QUICK scheme. Stability and accuracy of the results are maintained through a careful selection of the parameters involved in the models. The use of the conservation equation for energy is justified as an indirect means to evaluate the thermal field. Results are compared with experimental cases for validation purposes as well as LES computations for performance comparison and evaluation of the degree of model complexity needed to achieve satisfactory results.

2019-09-02Conference object

Open access

A study of a single droplet impinging onto a sloped surface: Jet-Fuel and Biofuel mixtures

Publication . Ferrão, Inês; Barata, Jorge M M; Silva, A. R. R.

The present study experimentally investigated droplets impinging on a sloped aluminum surface. In these experiments, the droplet is spherical throughout the trajectory where the gravitational acceleration and movement of the droplet have the same direction, which leads to unique phenomena. Considering an oblique impact, the droplet collides with a certain angle, and the impact velocity is composed of normal and tangential components to the surface that vary with the impact height. Four fluids were tested: 100% Jet-Fuel, 75% Jet-Fuel/ 25% HVO, 50% Jet-Fuel/ 50% HVO and H2O (pure water) as a reference. The mixtures were a combination of a conventional Jet Fuel (Jet A-1) and a biofuel (HVO – Hydroprocessed Vegetable Oil), more specifically NExBTL. When a droplet impacts onto an inclined surface, its shape is distorted and it can spread or splash asymmetrically relatively to the point of impact, affecting the advancing and receding contact angle. Therefore, several geometric parameters were measured and compared for the different fluids and incident angles. The incident angles influence the spreading velocity on the upper and lower side. The spreading velocity was analyzed, allowing a better understanding of the dynamic behavior of each side. The spread factor, which corresponds to the distance between the lower and upper edges normalized by the initial droplet diameter, was compared to the Weber number (We) which is a proper indicator for the drop deformation. The variation of incident angles and impact velocity promotes a different droplet movement and asymmetry in phenomena that are also evidently influenced by the gravity role.

2019-09-02Conference object

Open access

Propulsive Enhancement and Dynamic Stall Mitigation in Flapping Airfoils

Publication . Camacho, Emanuel António Rodrigues; Silva, André Resende Rodrigues da; Marques, Flávio Donizetti

The study of oscillating airfoils is the centerpiece for the biomimetization of flight, exploring newer energy-extracting devices, and improving rotor blade design. Typically, these are affected by highly nonlinear aerodynamic effects, which in most cases, still require substantial research regarding the aerodynamics and parameters that govern these systems. The present investigation looks into some of these topics by scrutinizing two primary fronts: the propulsive enhancement and the dynamic stall mitigation of flapping airfoils. After a comprehensive review of past investigations, the current work proposes a newer airfoil, the NACA0012-IK30 airfoil, that introduces the concept of dynamic curvature. This bioinspired concept is applied to the conventional NACA0012, producing an innovative geometry by dividing the airfoil into two parts, where we can deflect the leading edge independently from the rest of the airfoil. This newer design went through parametrical and optimal analysis by employing a diverse set of experimental and numerical methodologies, divided into two domains: lower Reynolds number, where the propulsive capabilities are studied, and higher Reynolds number, where dynamic stall mitigation is explored. At the lower spectrum of the Reynolds number, results show that the activation of the leading edge offered by the IK30 mechanism demonstrated the ability to improve considerably the propulsive power and efficiency. When comparing the proposed geometry with standard flapping, it is clear that the suggested design can provide optimal thrust with near-optimal propulsive efficiency, something unachievable in traditional flapping. Results obtained at the higher range of the Reynolds number, where the IK30 mechanism functions with dynamic stall mitigation purposes, show the adequacy of the proposed geometry under different plunging and pitching conditions. When correctly deflecting the leading edge, the aerodynamic stall experienced both in static and dynamic conditions can be mitigated or even eradicated, leading to significant drag reductions and modest lift enhancements. While the present study yielded encouraging results, the IK30 mechanism and the broad concept of dynamic curvature need to be the focus of further research. Using newer technologies, for instance, continuous camber morphing, coupled with ingenious kinematics, will allow us to explore newer pathways to extend our knowledge and exploit the aerodynamics of unsteady airfoils.

2024-11-08Doctoral thesis

Open access